There is disclosed a braking force control apparatus having a brake assist function and an antilock brake function in Japanese Patent Unexamined Publication No. 4-121260. The brake assist function (hereinafter, refer to as an ABS function) is provided in order to generate a braking hydraulic pressure greater than that at a normal time when an emergency braking operation is executed by a driver of a vehicle. The antilock brake function (hereinafter, refer to as an ABS function) is provided in order to control a wheel cylinder pressure P.sub.W/C in each wheel so that no excess slip ratio is generated in each wheel during an execution of the braking operation.
A conventional braking force control apparatus for realizing the ABS function is provided with a master cylinder for generating a braking hydraulic pressure in accordance with a brake pedaling force and a hydraulic pressure circuit for controlling a conductive state between the master cylinder and each wheel and a conductive state between a reservoir tank and each wheel. The hydraulic pressure circuit is controlled such as to communicate a wheel cylinder which is required to be increased the wheel cylinder pressure P.sub.W/C with the master cylinder and communicate a wheel cylinder which is required to be reduced the wheel cylinder pressure P.sub.W/C with the reservoir tank. In accordance with the control mentioned above, it is possible to properly control the wheel cylinder pressure P.sub.W/C of each wheel in a range lower than the braking hydraulic pressure generated by the master cylinder.
A conventional braking force control apparatus for realizing the BA function is provided with a high pressure source for generating a predetermined hydraulic pressure without relation to the brake pedaling force and a hydraulic pressure control valve for properly controlling a hydraulic pressure generated by the high pressure source in a pressure reduction manner so as to be supplied to the wheel cylinder in each wheel. The hydraulic pressure control valve supplies the braking hydraulic pressure increased at a predetermined number of force rate with respect to the brake pedaling force to each wheel in the case that the emergency braking operation is not executed by a driver. Further, the hydraulic pressure control valve supplied a brake fluid having a maximum hydraulic pressure generated by the high pressure source to the wheel cylinder in each wheel.
In accordance with the processing mentioned above, in the case that the normal braking operation is executed by a driver, it is possible to supply the wheel cylinder pressure P.sub.W/C in accordance with the brake pedaling force to the wheel cylinder in each wheel. Further, in the case that the emergency braking operation is executed by a driver, it is possible to supply the wheel cylinder pressure P.sub.W/C higher than that at the normal time to the wheel cylinder in each wheel. As mentioned above, in accordance with the conventional braking force control apparatus, it is possible to properly realize the function as the normal brake and the BA function.
As the braking force control apparatus for realizing both of the BA function and the ABS function, for example, there is considered an apparatus capable of selectively supplying the braking hydraulic pressure generated by the master cylinder and the braking hydraulic pressure generated by the high pressure source. In the braking force control apparatus as mentioned above, the ABS function can be realized by controlling the hydraulic pressure circuit in accordance with the method mentioned above while supplying the hydraulic pressure generated by the master cylinder to the hydraulic pressure circuit. Further, the BA function is realized by supplying the braking hydraulic pressure generated from the high pressure source to the wheel cylinder in each wheel via the hydraulic pressure circuit in a state of shutting off the master cylinder and the hydraulic pressure circuit. Hereinafter, in the braking force control apparatus mentioned above, a control for realizing the BA function and a control for realizing the ABS function will be respectively referred to as a BA control and an ABS control.
In the braking force control apparatus for realizing both of the BA function and the ABS function, when the BA control is started, there is a case that an excess slip ratio is generated in any one of the wheels. In this case, when the ABS control is executed with respect to the wheel, it is possible to simultaneously realize the BA function and the ABS function. The function mentioned above can be realized by controlling the hydraulic pressure circuit so that the wheel cylinder of the wheel in which the excess slip ratio is generated is properly connected to a side of the reservoir tank while supplying the braking hydraulic pressure generated from the high pressure source to the hydraulic pressure circuit. Hereinafter, the function mentioned above is referred to as a BA+ABS function, and a control for realizing the BA+ABS function is referred to as a BA+ABS control.
In accordance with the BA+ABS control mentioned above, after the emergency braking operation is executed by a driver, it is possible to increase a wheel cylinder pressure P.sub.W/C in the other wheels in response to the requirement of the BA control while increasing and reducing the wheel cylinder pressure P.sub.W/C of the wheel in which the excess slip ratio is generated in response to the requirement of the ABS control.
However, during the execution of the BA+ABS control, an interference is generated between the BA control and the ABS control. That is, during the execution of the BA+ABS control, an environment different from the case that the BA control is executed alone and the case that the ABS control is executed alone is formed. Accordingly, at a time of executing the BA +ABS control, there is generated the case of not properly controlling the wheel cylinder pressure PWlC when the BA control and the ABS control are executed in the same manner as that in the case they are executed alone.
That is, during the execution of the BA+ABS control, it is necessary to detach the wheel cylinder of a subject wheel for the ABS control (hereinafter, refer to as an ABS subject wheel) from the high pressure source except a little time at which an increase of the wheel cylinder pressure P.sub.W/C is required with respect to the wheel. On the other hand, the high pressure source is given a capacity sufficient for increasing the wheel cylinder pressures P.sub.W/C of all of four wheels in accordance with a proper increased pressure slope after the BA control is started. Accordingly, during the execution of the BA+ABS control, a change rate of the wheel cylinder pressure P.sub.W/C with respect to a non-subject wheel for the ABS control (hereinafter, refer to as an ABS non-subject wheel) becomes a steep increased pressure slope in comparison with the case that the BA control is executed alone.
Further, in the case that the ABS control is executed alone, the master cylinder pressure P.sub.W/C is supplied to the wheel cylinder of the ABS subject wheel. On the other hand, during the execution of the BA+ABS control, the hydraulic pressure generated by the high pressure source is supplied to the wheel cylinder of the ABS subject wheel. The high pressure source generates a hydraulic pressure higher than the hydraulic pressure normally generated as the master cylinder pressure P.sub.M/C. Accordingly, during the execution of the BA+ABS control, a change rate of the wheel cylinder pressure P.sub.W/C of the ABS subject wheel easily becomes a steep increased pressure slope in comparison with the case that the ABS control is executed alone.
The wheel cylinder pressure P.sub.W/C of the ABS subject wheel is reduced at a time when the excess slip ratio is generated in the wheel, and thereafter, is increased at a relatively slow degree. At a time of this pressure increase, if the wheel cylinder pressure P.sub.W/C is increased in accordance with the steep increased pressure slope, it is necessary to reduce the wheel cylinder pressure P.sub.W/C immediately after the pressure increase of the wheel cylinder pressure P.sub.W/C is started. Accordingly, if the wheel cylinder pressure P.sub.W/C is increased in accordance with the steep increased pressure slope as mentioned above during the execution of the BA+ABS control, a hunting under a control is easily generated with respect to the ABS subject wheel.
As mentioned above, in the case of intending to realize the BA+ABS function by the method mentioned above, there is a problem that the BA control and the ABS control are interfered with each other, so that the hunting under the control is easily generated in the ABS subject wheel and the excess increased pressure slope is generated in the ABS non-subject wheel (the subject wheel for the BA control). In this point, the method mentioned above is not always an optimum method in view of realizing the BA+ABS function.